The present invention relates to wound treatment and, in particular, it concerns a device and methods for accelerating wound healing and enhancement of muscle regeneration with microspheres as a therapeutic agent.
Wound healing is a complex process involving such factors as cells, extracellular matrix (ECM) components and the cellular microenvironment. Essentially, all wound healing involves the repair or replacement of damaged tissues. The precise nature of such repair or replacement depends upon the tissues involved, although all such processes involve certain basic principles. To illustrate these principles, cutaneous, or skin, wound healing will be described, it being understood that the discussion could also extend to all types of wound repair.
Skin has multiple layers, including keratin, epidermis and dermis. If only the epidermis is damaged, as in most minor injuries, keratinocytes migrate from the edge of wound and eventually cover it, reforming the epidermis and keratin D. R. Knighton and V. D. Fiegel, Invest. Radiol., 26:604-611, 1991!.
If all skin layers are damaged or destroyed, new connective tissue, called granulation tissue, must first fill the wound space. This tissue is formed by deposition of ECM components by fibroblasts, which migrate into the wound space D. R. Knighton and V. D. Fiegel, Invest. Radiol., 26:604-611, 1991!. The deposition of these ECM components, such as collagen, is currently believed to be important for healing of the wound. Indeed, the prior art teaches that the strength of the healing wound is ultimately dependent upon collagen deposition Haukipuro, K., et al., Ann. Surg., 213:75-80, 1991!. Thus, collagen deposition must be present at a sufficiently high level to give the healing wound strength and support.
This entire multi-step process must be completed for successful wound healing. If one or more of these components is missing, healing does not take place, the skin is not repaired and the wound remains open. Such open wounds can easily become infected, further retarding the process of healing and leading to the formation of ulcers and sores on the skin. The process of wound healing is further inhibited in many patients by the presence of other complicating conditions, such as diabetes and old age. Patients with such conditions often have skin wounds which ulcerate and refuse to heal, or only heal slowly after an extended period of time has elapsed.
Various treatments have been used in order to accelerate the rate at which wounds heal. For example, U.S. Pat. No. 4,772,591 discloses a method of accelerating the rate of wound healing by applying a combination of ascorbic acid, calcium, tyrosine or phenylalanine, and anti-inflammatory substances to the wound. Similarly, U.S. Pat. No. 4,590,212 discloses a method of applying acetaminophen to the wound. Many other patents have focussed upon other methods of accelerating the rate of wound healing. However, none of these methods has proven broadly effective.
In an attempt to improve treatments for wounds, various pharmaceutical carriers have been employed to deliver chemotherapeutic agents to the wound. Such carriers are particularly required for skin wounds since they are generally either exposed to air, or covered by bandages or clothing. In either case, the therapeutic agent can easily be removed by rubbing, for example. Thus, various creams, gels and powders have been used as pharmaceutical carriers, in an attempt to overcome this problem.
One interesting group of pharmaceutical carriers employs microspheres, which are small, microscopic particles made of various materials, including plastics and long-chain carbohydrates. Many prior art applications are known for microspheres as carriers for various therapeutic agents. For example, U.S. Pat. No. 5,264,207 discloses microspheres as a carrier for a pharmaceutical or cosmetic substance. A composition containing the microspheres and the active substance is applied cutaneously, with the microsphercs in effect enabling such a route of administration for the active substance. However, this reference does not teach or suggest using the microspheres themselves as a therapeutic substance.
Similarly, PCT Application Nos. WO 96/13164 and WO 94/13333 both disclose microspheres made of a material which catalyzes the production or release of certain therapeutic substances. PCT Application No. WO 96/13164 discloses polymeric nitric oxide adducts which release nitric oxide when directly applied to damaged tissue. PCT Application No. WO 94/13333 discloses particles which are chemically modified to have free radical activity in the wound environment. Again, neither reference teaches or suggests using the microspheres themselves as a therapeutic substance, without chemical modification of the microsphere material.
However, certain properties of the microspheres used as pharmaceutical carriers were shown to influence the effect of the therapeutic substance itself. For example, the activation of cytotoxic T lymphocytes by class I alloantigen immobilized on latex microspheres was studied. Although the class I alloantigen was clearly providing the stimulus itself, the extent of cell stimulation was increased by using particle sizes of 4 to 5 microns M. F. Mescher, J. Immunol., 149:2402-2405, 1992!. Such increased stimulation may demonstrate surface contact requirements for cytotoxic T lymphocytes. In other words, the optimum particle size may have increased the effect of class I alloantigen by providing an optimum surface area for cell contact. It should be emphasized, however, that these beads were still only carriers for the active substance.
Attempts have been made to exploit the apparent ability of certain particles to enhance the efficacy of active substances to promote wound healing. For example, U.S. Pat. No. 3,842,830 discloses glass microparticles which act to promote wound healing when directly applied to damaged tissue. U.S. Pat. No. 5,092,883 discloses biodegradable positively-charged dextran beads with a similar ability to promote osteogenesis and healing of soft tissue injuries. However, none of these references teaches or suggests the promotion of regeneration of muscle by administration of microspheres to the wound. Furthermore, none of these references teaches microspheres which initially promote more rapid cell metabolism and proliferation, yet which have a limited, finite effect, so that rapid cell metabolism and proliferation are not permanently induced.
Such a limited effect is especially important in promoting wound healing, which requires an initial increase in cell metabolism and proliferation, followed by a cessation of such cell activation after healing has occurred. Without an induction of such activation, wound healing will not occur. However, if cell activation does not cease after healing is substantially complete, abnormal scar formation can result, as in the formation of keloids. Thus, there must be a balance between promotion and inhibition of cell metabolism and proliferation during wound healing.
There is therefore an unmet medical need for a particulate substance which can be directly applied to damaged tissue in order to promote healing, yet which has self-limited effects and which is substantially non-toxic, and which can also promote muscle regeneration.